The invention is in the field of light-emitting semiconductor devices such as light-emitting diodes (LEDs) and lasers, and relates more particularly to metal electrical contacts for such devices.
Presently, LED devices having GaN contact layers are typically provided with metal electrical contacts of an alloy of nickel and gold. A representative LED device having GaN contact layers and metal electrical contacts of a nickel-gold alloy are shown in U.S. patent application Ser. No. 08/991,522 of Taskar et al, filed Dec. 16, 1997, and incorporated herein by reference in its entirety. While such nickel-gold alloy electrical contacts are entirely suitable for light-emitting semiconductor devices operating at low current density, and in fact are advantageous due to their relatively low resistance, they are not suitable for use in high-power light-emitting devices where high current densities are encountered. This is because such high-power, high current density devices typically operate at higher temperatures, at which the nickel-gold contacts will cause device failure by causing a short circuit in the device. It is believed that the nickel-gold contacts short the p-n junction of the device by migration of nickel or nickel compounds through defects within the GaN contact layer material at relatively low temperatures.
In order to overcome this deficiency, experiments have been conducted with metal electrical contacts formed of titanium-gold alloys, which have been found to function at an intermediate temperature at which nickel-gold alloy contacts would typically short, with the disadvantage of a somewhat greater contact resistance. Additionally, since titanium-gold alloy contacts will still cause short circuits at temperatures lower than those required for operation by high current density, high-power devices, a substantial need exists for a metal electrical contact composition which can successfully operate at high current densities and high temperatures without creating a short circuit in the light-emitting semiconductor devices in which they are employed.